Optical devices that control the routing of individual optical signals can enhance the functionality of an optical network by dynamically reconfiguring the optical network. Accordingly, a number of devices have been developed to control the routing of optical signals, particularly between different optical fibers. For example, U.S. patent application Ser. No. 6,324,316, entitled “Fabrication Of A Total Internal Reflection Optical Switch With Vertical Fluid Fill-Holes” to Fouquet et al. and U.S. Pat. No. 6,195,478, entitled “Planar Lightwave Circuit-Based Optical Switches Using Micromirrors in Trenches” to J. Fouquet describe optical switches that control routing of optical signals to and form optical fibers. These particular optical switches employ sealed cavities that are between integrated circuits and optical plates.
FIG. 1A shows a cross-sectional view of an optical switch 100 that includes an integrated circuit 110 and an optical plate 120. Integrated circuit 110 contains active circuitry that operates switching sites in optical switch 100 to control the routing of optical signals passing through optical switch 100. Optical plate 120 carries the optical signals and is made of an optical material such as fused silica (quartz) in which waveguides have been formed.
A seal 130 that attaches optical plate 120 to integrated circuit 110 holds optical plate 120 away from the active surface of integrated circuit 110 to thereby form a cavity 140 between integrated circuit 110 and optical plate 120. As shown in FIG. 1B, seal 130 predominantly follows the perimeter of integrated circuit 110 so that the active area on integrated circuit 110 is inside cavity 140. Accordingly, seal 130 and cavity 140 inherit the rectangular shape of integrated circuit 110.
For operation of optical switch 100, cavity 140 is filled with a fluid, and seal 130 must hermetically seal cavity 140 to prevent contamination of the fluid, keep the fluid in cavity 140, and provide optical switch 100 with an acceptable useful life. Making cavity 140 or any cavity truly hermetic is not currently possible. Manufacturers of ultra-high vacuum equipment are well aware that small molecules in gases such as helium can diffuse at appreciable rates though inches of solid steel. Hermeticity is therefore a relative measure of the leak-tightness of an enclosure, and the maximum acceptable leakage rate depends on the application.
For optical switch 100, cavity 140 can tolerate a “hermetic” seal with a leakage rate of about 1×10−9 Pa·m3/sec or lower. Integrated circuit 110 and optical plate 120 are dense materials and therefore have an intrinsic leakage rate below 1×10−15 Pa·m3/sec. The quality of seal 130 therefore controls the hermeticity of cavity 140.
Plastic and other organic materials, which might otherwise be suitable for seal 130, generally have open molecular structures that are unable to achieve leakage rates better than 1×10−8 Pa·m3/sec. Ceramics and glasses may be able to achieve the desired leakage rate but generally do not have the thermal and mechanical properties required for a durable seal. Neither do they have favorable characteristics for manufacturing a seal. Seal 130 is therefore preferably metallic.
For metal seal 130 to be hermetic, the metal must be sufficiently thick to present a fully dense barrier to diffusing gas, and the metal must form leak-free unions with the surfaces of the abutting components. Metal a few tens of microns thick in the direction perpendicular to the gas diffusion can provide the required diffusion barrier. Forming leak-free unions of metal seal 130 with optical plate 120 (quartz) and with integrated circuit 110 (silicon) generally requires that the areas of optical plate 120 and integrated circuit 110 be metalized. Making seal 130 is then a metal-to-metal joining problem.
Some of the options for metal-to-metal joining are solid phase welding, liquid phase welding, brazing, and soldering. Each of these processes has its merits and limitations. However, soldering is the most practical joining method because electrical fields, excessive temperature, and localized mechanical stress resulting from other metal joining processes can damage integrated circuit 110.
Selection of the most appropriate solder for seal 130 depends largely on the temperature limitations of the joining process and on the thermal and mechanical properties of the solder and the joined materials. All materials expand and contract, to a greater or lesser degree, with changes in temperature. The coefficient of thermal expansion (CTE) indicates the rate of expansion of a material with temperature and is typically expressed in units of ppm/° K. Quartz such as used in optical plate 120 has a CTE of about 0.5 ppm/° K and does not alter its physical size much over the normal temperature range for switch 100. Silicon in integrated circuit 110 has a CTE of approximately 2.5 ppm/° K, making the differential CTE between optical plate 120 and integrated circuit 110 about 2 ppm/° K.
Hermetic seal 130 must be able to absorb the resulting differential expansion and contraction of integrated circuit 110 and optical plate 120 if the temperature of optical switch 100 varies. Telecordia specifications, which are defined for all photonics parts and equipment, require that seal 130 remain intact on cyclic exposure to temperatures between −40° C. and +85° C. Ceramics and glass do not have sufficient ductility to accommodate the resulting physical displacement and therefore are not suitable seal materials. The preferred seal materials, solders, are also not well suited to this application because thermo-mechanical mechanisms can cause soldered joints to fail when subjected to cyclic thermal and/or physical stress. This is because solders do not have significant elastic ranges, so that essentially all deformation is plastic. Solders subject to constant stress thus fail by creep rupture, and solders subject to cyclic stress fail by fatigue. In both cases, the failure mechanism is microscopic structural changes that result in the formation of voids that coalesce to form fracture surfaces.
The traditional methods of circumventing these problems with solder seals have involved either designing products to minimize the net CTE mismatch or increasing the joint thickness to improve the stress distribution. An alternative approach is use of the most robust solder possible in order to buy the longest possible life. However, the so-called “hard solders” (e.g., Au-20Sn) invariably have high melting points, and their mechanical stiffness can distort joined components during cooling from the joining temperature.
In view of the drawbacks of current seals, improved seals and sealing methods are sought.